Laser Spectroscopy of Be-like Krypton @ ESR

High-resolution laser spectroscopy is a very precise method for investigations of the atomic structure, being sensitive to the smallest effects, such as QED, relativity, and electron-electron correlations. In order to study correlation effects in the strong electromagnetic fields of heavy highly-charged ions, Be-like systems (with four electrons) are ideal candidates. By comparing state-of-the-art atomic structure calculations with new experimental data, correlation effects could be tested on the percent level. In February 2020, the GSI facility produced, accelerated and stored Be-like krypton ions (86Kr32+) in the ESR for a test beam time of experiment E135:

Laser spectroscopy of the (1s2 2s2p) 3P0 - 3P1 level splitting in Be-like krypton (G-PAC, A-).

During the test beam time, the main components of the experiment have been tested using the real ion beam in the ESR. These components are: two pulsed UV laser systems, an in vacuo moveable XUV fluorescence detector, a new HV divider (for the E-Cooler voltage), and the improved DAQ system. Laser spectroscopy was performed in the anti-collinear geometry (laser beam counter-propagates ion beam), so that the ≈10.4 eV 3P0 - 3P1 transition energy could be reached using UV laser beams (257 nm and 276 nm) by exploiting the huge Doppler shift from the stored relativistic ions (b ≈ 0.69). The data acquired during the test experiment are currently still under analysis and it is not yet clear if the transition has indeed been found. Nevertheless, we will report on the experiment itself and on the current status of the analysis, showing some preliminary results. We will also comment on further improvements for the real experiment, which can hopefully be performed during a next beamtime @ ESR (2021-2022).